Neuromuscular blocking agents are commonly given intravenously during general anesthesia to relax the muscles, intubate the trachea and facilitate controlled ventilation.
At present, two types (classes) of neuromuscular blocking agents predominate in clinical practice:
(1) Neuromuscular blocking agents of intermediate duration. Examples are rocuronium, vecuronium, and cisatracurium. This type of agent is commonly given by repetitive bolus for maintenance of neuromuscular blockade, and may also be given in high dosage to facilitate intubation of the trachea. Recovery from neuromuscular blockade is relatively slow and commonly requires antagonism (reversal) by administration of an anticholinesterase, such as neostigmine, at the end of the anesthesia, to restore normal function within a reasonably short time (10-15 min).
(2) Neuromuscular blocking agents of short or ultra-short duration. Examples are mivacurium (short) and succinylcholine (ultra-short). This type of agent commonly displays a fast onset of effect and is given to facilitate intubation of the trachea. A second convenient and safe usage is for maintenance of blockade by continuous infusion, where spontaneous recovery occurs rapidly on discontinuation of the infusion.
Both the above classes of neuromuscular blocking agents have disadvantages. Slow recovery from the intermediate-duration agents requires administration of the antagonist neostigmine. The relatively slow onset of blockade of this type (class) of neuromuscular blocking agents requires high dosage to enable tracheal intubation within 60-90 seconds. This high dosage lengthens blockade considerably, to as much as one to three hours or more, during which time the block may be too deep to be antagonized by neostigmine.
The short and ultra-short acting agents mivacurium and succinylcholine are both metabolized by pseudocholinesterase, an enzyme system which may be abnormal for a wide variety of reasons; in such cases, the blockade produced by these drugs is markedly prolonged.
Though it is still in common usage because of its fast onset which facilitates early (60 sec) intubation of the trachea, succinylcholine has many other well-known undesirable side-effects such as cardiac arrhythmias and muscle pain, and the drug is a triggering agent for the often fatal malignant hyperthermia syndrome. One of the disadvantages of mivacurium is its histamine releasing property which, though relatively weak, requires improvement such that it is reduced or abolished entirely. Further, the onset of mivacurium-induced block is comparatively slow vis-a-vis succinylcholine.